This invention pertains to improving the sensitivity and B.sub.1 homogeneity of NMR and MRI experiments, particularly on large samples at high fields, by means of novel, low-inductance "saddle" or "volume" coils for use on surfaces usually aligned with B.sub.0. Related NMR coils are described by Zens in U.S. Pat. No. 4,398,149, Hill and Zens in U.S. Pat. No. 4,517,516, and Doty in U.S. Pat. No. 4,641,098. Sensitivity optimization is reviewed by David Doty in "Probe Design and Construction" in The NMR Encyclopedia, Vol. 6, Wiley Press, 1996, and numerous coils are reviewed by James Hyde in "Surface and Other Local Coils for In Vivo Studies", Vol. 7, of the same series. A separate application discloses improved surface coils using litz foil.
The novel coils in this invention have higher quality factor Q, filling factor .eta..sub.F, homogeneity of the transverse magnetic field B.sub.1, and self-resonance f.sub.0. They may also be designed to have sharper roll-off in B.sub.1 beyond the region of interest. In high resolution NMR, this gives improved NMR lineshape by reducing sensitivity to sample material located in regions of poor B.sub.0 homogeneity, a problem previously addressed by Zens in U.S. Pat. No. 4,549,136 using susceptibility-matched plugs and by Hill in U.S. Pat. No. 4,563,648 using geometric compensation and by many others using shielding (Ad Bax, J. Magn. Reson. 65, pp. 142-145, 1985). In MRI, the improved roll-off reduces foldback from signals originating near the gradient null point.
The improvements in this invention are achieved primarily from the use of judiciously chosen, circuitous, parallel, current paths with insulated crossovers that result in (1) greatly reduced current density (typically a factor of three below that of prior-art designs of comparable inductance) along the inner edges of the conductor bands which subtend small angles with respect to the B.sub.1 direction, (2) transparency to self-generated B.sub.1, and (3) to a lesser extent, improved transparency to orthogonal rf fields. Transparency to self-generated rf fields in low-inductance coils has generally required phase-shift networks, as disclosed by Edelstein et al. in U.S. Pat. No. 4,680,548, but their birdcage design is extremely difficult to tune over a wide range of sample loading conditions or even over the narrowest of multi-nuclear ranges, such as .sup.19 F-.sup.1 H, a 6% change in frequency. Transparency to self-generated fields can also be achieved by other distributed parameter resonators, as disclosed by Hayes et al., U.S. Pat. No. 4,783,641, and again in U.S. Pat. No. 5,053,711, but these resonators all have severely limited tuning range and inferior B.sub.1 homogeneity. Moreover, the ring currents in the birdcage and induced currents in the external rf shield result in poor rf homogeneity ("hot spots" and "dark regions") for large samples.
The NMR spectroscopist often finds it necessary to observe a wide variety of nuclides, especially .sup.13 C, .sup.1 H, .sub.19 F, .sup.27 Al, .sup.29 Si, .sup.23 Na, .sup.2 H, and .sup.15 N in the study of commercially and scientifically important chemicals, and considerable interest is developing in multi-nuclear localized MR spectroscopy. Multi-nuclear tuning is readily achieved in prior art designs with sample diameters up to 15 mm at high field with multi-turn coils having inductance typically in the range of 35 to 150 nH, while fixed-frequency coils may have inductance as low as 2 nH.
Recent improvements by Kost et al. (and proprietary designs by Varian and Nalorac) in the conventional slotted-resonator have improved its homogeneity, but it is still often not adequate for large samples when the ratio of the diameter of the external rf shield to the coil diameter is less than approximately 1.5. Moreover, for coil diameters below 20 mm, its inductance is usually too low for efficient, broad-band, multi-nuclear tuning, although it performs reasonably well for fixed frequencies or narrow-band tuning. Because the conventional 2-turn saddle coil has about four times the inductance of the slotted resonator and there have been no good options in-between, one is often faced with the dilemma of not being able to fabricate a coil with optimum inductance for the size and frequency needed.
Phased-array coils, as disclosed first by Carlson in U.S. Pat. No. 4,857,846, consisting of typically four or more separately tuned surface coils, are generally designed for minimal coupling coefficients between adjacent resonators. This offers advantages in localized sensitivity compared to volume coils (though individual surface coils are generally superior in this regard), but at enormous price in tuning complexity and reduced loading flexibility. Nonetheless, phased arrays have become quite popular, possibly because of the lack of highly flexible, multi-purpose local coils. In practice, surface coils are always made and elaborately packaged for specific applications such as shoulder, rear neck, front neck, face, breast, lumbar, etc. The instant invention permits the development of tunable wrap-around coils that are likely to replace surface coils in many applications.
Carlson has also shown in U.S. Pat. No. 4,878,022 that reducing the current concentrations at the edges of coils results in reduced losses in both the sample and the coil.
Isaac et al. and Fitzsimmons et al. have devised complex methods of double-tuning the birdcage, but satisfactory commercial products are generally not available. Bolinger et al. have attempted to improve B.sub.1 homogeneity in large single-turn and half-turn tunable structures that resemble the birdcage without phase shifting networks by adjusting the spacing of parallel wires, but they fail to fully consider self-induced currents, ring currents, and shield currents; and their design suffers from poor filling factor and poor B.sub.1 homogeneity near the conductor elements. Most of the inventive embodiments illustrated herein may be easily double and triple tuned by conventional methods, similar to those, for example, of Doty in U.S. Pat. No. 5,424,625.
One of the most recently introduced fast MRI techniques, Pi-Echo-Planar-Imaging (PEPI, P. Mansfield), promises substantial improvements in resolution by virtually eliminating the susceptibility artifacts that plagued prior multi-scan EPI techniques, but it places extremely stringent demands on B.sub.1 homogeneity. The instant invention achieves order-of-magnitude improvement in B.sub.1 homogeneity for large samples when compared to the 8-rung birdcage with a shield-to-coil diameter ratio of 1.2, and it even compares favorably with the unshielded 24-rung birdcage with perfect (impossible) tuning.
The linearly polarized embodiments of the instant invention provide substantially higher homogeneity than any previous low inductance transverse coil design, simple tune-up, high sensitivity, and excellent line shape. Since they do not have well-defined degenerate modes, they are not inherently quadrature coils, but their rf transparency allows two such coils to be aligned orthogonally at slightly differing radii and easily tuned to the same frequency for quadrature operation. However, linearly polarized coils generally have comparable S/N and numerous advantages for low frequency-diameter products. For example, they greatly simplify the use of separate coils for transmit-receive or double resonance. Circularly polarized embodiments are also disclosed, but they are usually less satisfactory than the combination of two linearly polarized coils.
The above-referenced prior art is generally directed at applications in which the coilform is a cylinder aligned with the external magnetic field. Those cases where the sample axis is not aligned with the external magnetic field have generally utilized solenoidal rf coils. However, saddle coils have occasionally been used for the Dynamic Angle Spinning (DAS) line narrowing technique for quadrupolar nuclides in solids and related experimental techniques for dipolar nuclides, in which a solid sample is spun rapidly at a sequence of two or more angles ranging from 0.degree. to 90.degree.. The conventional saddle coil has the advantage that even though its efficiency is less than that of the solenoid for coil orientation angles greater than roughly 35.degree.(which includes the popular "magic angle" of 54.7.degree. and the popular "wideline" angle of 90.degree.), its efficiency is constant with angle while the efficiency of the solenoid approaches zero at 0.degree.. The advantages of the instant invention will reduce the range of applications where the solenoid is chosen for coils not aligned with the external magnetic field.
Litz (woven) wire, comprising multiple strands of insulated wire woven together, has long been used in rf coils to achieve higher Q, as it allows the current to distribute more uniformly over more surface area, especially in regions of high field gradient. Similar principles may be applied to NMR coils by using "woven" or "litz" foil, as space constraints generally favor the use of thin foil in NMR or MRI. Vujcic et al. disclose the use of litz wire for enhanced Q in high-inductance multi-turn birdcage-like resonators for low frequencies in Magn. Reson. in Med., Vol. 36, 1996, pp. 111-116. However, they do not consider deliberate spacings of the insulated strands to improve B.sub.1 homogeneity, Q, filling factor, flux transparency, or tunability. Flux through their litz wire is estimated to be approximately 1% of the total rf flux. Parallel helical wires without insulated crossovers have been used commercially by the assignee in solenoidal coils for enhanced Q and flux transparency, and solenoidal litz coils have been used elsewhere.
Even more dramatic than the Q advantage is the B.sub.1 homogeneity advantage that is possible because of the transparency to rf fields that arises from properly interleaved foils made by a combination of folds, twists, and jumpers. There is also typically a 20% to 40% improvement in filling factor, partially because a high-homogeneity coil may be positioned closer to the sample, and partially because the field concentrations near the wires are dramatically reduced. In applications where sample losses dominate, conventional guidelines say that filling factor is insignificant. However, both the improved filling factor and the improved B.sub.1 homogeneity contribute to reduced sample losses, especially by reducing induced current density in the vicinity of the conductors.
The various embodiments disclosed herein and in copending applications are applicable to perhaps 90% of all NMR and MRI volume coil applications and many surface coil applications, including (a) 3 mm to 15 mm .sup.1 H and multi-nuclear NMR high-resolution spectroscopy at 4 T to 30 T, (b) 2 mm to 60 mm .sup.1 H and multinuclear MR microscopy at 2 T to 12 T, (c) head and whole-body MRI at least up to 3 T, (d) MRI wrap-around (torso) resonators at least up to 7 T, and (e) most surface coils. The result is better performance in broadband tunable coils than was previously possible in fixed-frequency coils.